1. Field of the Invention
This invention relates to an exhaust gas purification system of an internal combustion engine.
2. Description of the Related Art
Internal combustion engines are ordinarily provided with a catalytic converter comprising a three-way catalyst in the exhaust system which removes HC, NOx and CO components in the exhaust gas generated by the engine. However, when the catalytic converter is not activated, for example, at the time of engine coldstarting, unburned components of the exhaust gas including unburned HC are released immediately into the atmosphere.
For that reason, as taught by Japanese Laid-Open Patent Application No. 10 (1998)-153112, there has been proposed an exhaust gas purification system which has an adsorbent made of a zeolite material or some similar material installed in a bypass exhaust gas passage branched from the exhaust pipe at a location downstream of the catalytic converter, which again merges into the exhaust pipe at a downstream point and has a switch-over valve which opens or closes the bypass exhaust gas passage. The switch-over valve opens the bypass exhaust gas passage when the catalytic converter has not been activated at cold engine start to introduce the exhaust gas such that the adsorbent adsorbs unburned components and closes the bypass exhaust gas passage such that the adsorbent desorbs the adsorbed components. The desorbed components are thereafter recirculated at a position upstream of the catalytic converter, e.g. to the air intake system, through a recirculation pipe after the catalytic converter has been activated.
In this kind of exhaust gas purification system, when the recirculation pipe is configured to connect the bypass exhaust gas passage to the air intake system, since the recirculation pipe becomes relatively long, gas in the pipe may sometimes condense into liquid water due to the ambient temperature and remain in the pipe. If this happens, the residual water could freeze to block the pipe or could corrode the pipe or the switch-over valve.